Specific Aims A. To determine the efficacy of an intense resistive exercise protocol or a daily dose of 10 mg of alendronate to prevent disuse bone loss, as assessed by: 1. Bone mineral densitometry of the whole body, spine, hip, calcaneus, wrist, and subregions of the whole body scan 2. Calcium balance 3. Metabolic bone markers: total and bone specific alkaline phosphatase, osteocalcin, total and ionized serum calcium, hydroxyproline, pyridinium crosslinks, deoxypyridinoline crosslinks 4. Bone regulating hormones: PTH and vitamin D 5. Non-invasive measures of bone quality (ultrasound and low frequency acoustic vibration) 6.Renal stone profiles B. To determine the efficacy of an intense resistive exercise protocol to prevent disuse muscle loss, as assessed by: 1. Muscle volume of the calf, thigh, lower back, neck 2. Muscle strength of the ankle, knee, back 3. Lean tissue mass of the whole body, arms, trunk, legs Alendronate and resistive exercise both look promisingl as countermeasures to bone loss during bed rest and therefore in space flight. Proper selection and execution of exercises is critical to the success of an exercise countermeasure in space flight. To complete this study, we plan to recruit 11 more subjects over the next 12 months. Two of these will be assigned to the exercise group, 5 to alendronate and 4 as controls.